You can check if a PO file is valid with the following command:

        msgfmt -c -o /dev/null <po file>
      

The following errors were found in mate-applets 's PO files:

gettext: accessx-status/docs/zh-Hans/zh-Hans.po: can't guess language
gettext: battstat/docs/zh-Hans/zh-Hans.po: can't guess language
gettext: charpick/help/zh-Hans/zh-Hans.po: can't guess language
gettext: cpufreq/help/zh-Hans/zh-Hans.po: can't guess language
gettext: drivemount/help/zh-Hans/zh-Hans.po: can't guess language
gettext: geyes/docs/zh-Hans/zh-Hans.po: can't guess language
gettext: mateweather/docs/zh-Hans/zh-Hans.po: can't guess language
gettext: multiload/docs/zh-Hans/zh-Hans.po: can't guess language
gettext: netspeed/help/zh-Hans/zh-Hans.po: can't guess language
gettext: stickynotes/docs/zh-Hans/zh-Hans.po: can't guess language
gettext: trashapplet/docs/zh-Hans/zh-Hans.po: can't guess language
      

Please ask the translator (identified by the Last-Translator field in the PO file), the language team (identified by the Language-Team field) or debian-i18n for a fix.